Polycarbosilane is a polymer having a skeletal structure composed substantially of recurring units expressed by the formula ##STR2## Since this polymer changes to silicon carbide (SiC) by firing, it is used as a raw material for the production of SiC which is utilized as fibers, films, coatings, sintering agents, impregnating agents, powders, etc.
Known polycarbosilanes include two species, one being polycarbosilane prepared by polymerizing monosilane, and the other being polycarbosilane obtained by converting monosilane to polysilane and polymerizing it. The former can be produced by the method disclosed, for example, in Fritz, Angew, Chem. 79, p. 657 (1967), and the latter can be produced by the method disclosed in U.S. Pat. No. 4,052,430 to the same inventors as the present application. Production of polycarbosilane having superior thermal stability and oxidation resistance and a high residual ratio on firing in a non-oxidizing atmosphere by conventional methods requires the use of a pressure reactor such as an autoclave or a circulating-type apparatus which permits recycling. According to a method utilizing the pressure reactor, the reaction must be carried out at a temperature of 400.degree. to 470.degree. C. and a pressure of 80 to 110 atmospheres for a period of 10 to 15 hours, and the provision of a pressure-resistant equipment and measures against a danger of fire are essential. Another defect is that this method is not suitable for mass production. According to a method using the circulating-type apparatus, it is necessary to use an apparatus including a heat reaction tower, a product-separating tower, etc., and low-molecular-weight products must be forcibly recycled for repeated reaction in the heat reaction tower. Thus, the temperature must be raised to as high as 600.degree. to 800.degree. C., and the reaction time must be as long as 20 to 50 hours. The latter method is therefore industrially disadvantageous in many respects.
The present inventors made extensive investigations to remove the aforesaid defects of the prior art, and consequently discovered a novel and advantageous process for producing polycarbosilane from polysilane at low heating temperatures within relatively short periods of time without the need for any special apparatus such as a pressure vessel or a recycle-type apparatus. The present inventors also found that the polycarbosilane obtained by this process is a novel polymer having better thermal stability and oxidation resistance and a higher residual ratio on firing in a non-oxidizing atmosphere than the polycarbosilanes obtained by the aforesaid prior methods.
The present inventors also found that this polycarbosilane is a very suitable material for silicon carbide fibers.
In the spec fication of U.S. Patent Application Ser. No. 677,960 filed on Apr. 19, 1976, the present inventors disclosed a process for producing silicon carbide fibers from polycarbosilane obtained by a prior art method, which comprises removing low-molecular-weight compounds from polycarbosilane to reduce the content of these compounds to 10% or less, preparing a spinning dope of the polycarbosilane, spinning the dope to form fibers, subjecting the fibers to a treatment of rendering them infusible, and firing the treated fibers at high temperatures to convert them to silicon carbide fibers. The fibers obtained by spinning in this process are so brittle that they are difficult to handle. The brittleness of these fibers can be reduced to some extent by rendering them infusible, but their tensile properties are still poor with a tensile strength of less than 3.0 kg/mm.sup.2 and a break elongation of 2%. As a result, breaking of the fibers tends to occur frequently, and the infusible fibers are difficult to handle. Moreover, breaking of fibers tends to occur in the final firing step. In contrast, when silicon carbide fibers are produced from the polycarbosilane obtained by the process newly discovered by the present inventors which have superior thermal stability and oxidation resistance and a high residual ratio on firing in a non-oxidizing atmosphere, the above defects of the prior art methods can be markedly remedied, and because of the high residual ratio on firing of the polycarbosilane, the yield of the silicon carbide fibers increases greatly. In addition to this advantage, the process of this invention can permit the omission of a step of removing low-molecular-weight compounds which is essential in the prior art methods. This brings about the advantage that silicon carbide fibers can be produced economically.
As stated above, the process for producing polycarbosilane discovered by the present inventors which does not require a pressure vessel or a circulating-type apparatus has many advantages. If required, however, polycarbosilane may be produced from polysilane by using a pressure vessel in this process. The present inventors have found that polysilane whose side chains bonded to the silicon atom consist of 1 to 50% of phenyl groups and the balance being methyl groups is an especially preferred starting material in this case.